The focus of our research program is to understand the role of coaggregation in bacterial accretion of early colonizing bacteria on a clean tooth surface. The primary colonizers include actinomyces, streptococci, and veillonellae. The adhesion that mediates lactose- inhibitable coaggregation between Veillonella atypica PK1910 and human oral Streptococcus spp. has been identified as a 45 kDa-protein. It is selectively eluted by lactose from agarose-lactose beads as well as from cells of Streptococcus oralis 34, a veillonella coaggregation partner. Antisera that react with this veillonella protein also block specifically the lactose-inhibitable veillonella-streptococcus coaggregations, while lactose-noninhibitable coaggregations are unaffected. The 38 kDa-protein from S. gordonii PK488 appears to be an important surface molecule for early colonizing streptococci. All streptococci that express an immunologically cross-reactive protein, so far tested in our laboratory, coaggregate width A. naeslundii PK606. The gene encoding this protein in one of these streptococci, S. sanguis 12, has been cloned in E. coli and has been sequenced. The functionally equivalent gene from S. gordonii PK488 has been cloned in E. coli by constructing a lambda expression library of strain PK488 chromosomal DNA. This library was screened immunologically and a clone expressing the 38 kDA-protein was identified. While intergeneric coaggregation among oral bacteria is commonplace, intrageneric coaggregation among oral bacteria is highly unusual, except among streptococci. We have initiated studies aimed at developing a genetic system among the streptococci. We have shown that plasmids can be naturally transformed and transformed by electroporation, and we have shown that conjugative transposons can be transferred among oral streptococci.